Directional composites by solid-state up-transformation

ABSTRACT

A cellular precipitation type alloy is heated to a temperature above its solvus temperature to form the high temperature single solid phase which is then quenched to retain it at about room temperature. The quenched single phase solid is heated unidirectionally through a thermal gradient at a velocity of transformation which on reaching transformation temperature directionally transforms it into an aligned composite of at least two phases with one of the phases aligned in the form of substantially uniform lamellae, fibers or rods substantially parallel to each other and to the thermal gradient. The transformation temperature is a minimum of 20*C below that of the normal down-temperature technique at the same velocity in a certain velocity of transformation range resulting in significantly finer microstructures.

United States Patent [191 Livingston 5] Oct. 29, 1974 [75] inventor: James D. Livingston, Scotia, NY.

[73] Assignee: General Electric Company,

Schenectady, NY.

[22] Filed: Nov. 15, 1973 [2]] Appl. No.2 416,254

[52] US. Cl 148/2, 148/3, 75/129, 75/135 [51] Int. Cl. C22c 19/00, C220 39/20 [58] Field of Search 148/2, 3, 4, I34, 135; 75/135, 129

[56] References Cited UNITED STATES PATENTS 3,552,953 l/l97l Lemkey et al. 75/l7l 3,635,769 l/l972 Shaw 75/l7l X 3,671,223 6/1972 Thompson et al. 75/135 X 3,677,835 7/l972 Tien et al 75/l35 X 3,783,033 l/l974 Tarshis 75/l70 X Primary Examiner-L. Dewayne Rutledge Assistant Examiner-Arthur J. Steiner Attorney, Agent, or Firm-Jane M. Binkowski; Joseph T. Cohen; Jerome C. Squillaro [5 7 ABSTRACT A cellular precipitation type alloy is heated to a temperature above its solvus temperature to form the high temperature single solid phase which is then quenched to retain it at about room temperature. The quenched single phase solid is heated unidirectionally through a thermal gradient at a velocity of transformation which on reaching transformation temperature directionally transforms it into an aligned composite of at least two phases with one of the phases aligned in the form of substantially uniform lamellae, fibers or rods substantially parallel to each other and to the thermal gradient. The transformation temperature is a minimum of 20C below that of the normal down-temperature technique at the same velocity in a certain velocity of transformation range resulting in significantly finer microstructures.

2 Claims, No Drawings DIRECTIONAL COMPOSITES BY SOLID-STATE UP-TRANSFORMATION The present invention relates generally to the art of directional control of cellular precipitation of metal alloys, and particularly, it relates to producing directional composites of cellular precipitation type alloys, i.e., alloys with one phase aligned in a matrix of a second or other phases.

The art has disclosed the directional transformation of eutectic and cellular precipitation type alloys to produce aligned composite structures. These transformations were accomplished by moving the alloy sample down a temperature gradient, i.e., moving the sample from a hot zone to a cold zone at a certain rate. Specifically, in cellular precipitation type systems the transformation occurs by cooling from the high temperature single phase solid through the solvus temperature below which transformation into at least two solid phases occurs. However, this normal down temperaturetransformation techniques as a means of producing aligned structures of most cellular precipitation type alloys has a major limitation and that is that it is too slow to be of practical value. Also, the relatively high temperatures at which precipitation occurs lead to a minimum attainable laminar spacing which is too coarse to provide the necessary properties for a number of applications.

The present invention overcomes the aforementioned limitations and provides in a certain velocity of transformation range a lower temperature of transformation at the transformation interface or front which leads to finer aligned microstructures. In addition, the present invention provides a velocity of transformation which is significantly faster than that attainable by the normal down temperature-transformation technique.

Briefly stated, the process of the present invention comprises providing an alloy which forms at an elevated temperature a single solid phase that precipitates a second solid phase when cooled through a solvus tem perature, said alloy having a precipitation temperature that varies with velocity of motion down a temperature gradient, heating said alloy to a temperature above the said solvus temperature to form said single solid phase, quenching the resulting single solid phase alloy to retain said phase at about room temperature, unidirectionally raising the temperature of the resulting quenched single solid phase alloy through a thermal gradient of at least 50C per cm. at a velocity which on reaching transformation temperature directionally transforms said quenched solid to produce an aligned solid composite of at least two phases wherein one of said phases is aligned in the form of lamellae, fibers or rods substantially parallel to each other and to the thermal gradient, said aligned phase being substantially uniform in size and passing through a matrix of the second or other phases, said transformation temperature being a minimum of 20C below said precipitation temperature when said velocities are equivalent in the range from a minimum velocity up to 75 percent of the maximum velocity of transformation to produce said aligned solid composite.

Since the transformation temperature in the present process is at least 20C below that of the precipitation or transformation temperature of the normal down temperature transformation technique, the aligned phase in the resulting composite is at least percent finer in size that that attained by the down temperature transformation technique. Also, with decreasing temperatures of transformation in the present process the aligned phase is correspondingly finer in size.

The present process uses an alloy which undergoes a cellular precipitation. Specifically, it is an alloy which forms a single solid phase at an elevated temperature, sometimes referred to as the high temperature single phase, and which, when cooled through a solvus temperature, precipitates a phase resulting in at least a two phase solid alloy. By a cellular precipitation type alloy it is meant one which can be directionally aligned to grow the precipitated phase as lamellae, rods or fibers substantially perpendicular to the transformation interface or front, i.e., substantially parallel to the thermal gradient. Representative of such alloys are Fe-Zn, Ni- Cr, Pb-Sn, Au-Ni, Ag-Cu, and Nb-Cr.

ln carrying out the present process, the solid alloy, preferably in the form of an ingot, is heated to a temperature above its solvus temperature to form the high temperature single solid phase. The formation of this high temperature single phase is determinable empirically by standard metallographic techniques. Also, the solvus temperature for a particular alloy is usually available in the literature.

The single phase alloy is then rapidly quenched to re tain it at about room temperature. A number of conventional methods can be used to carry out the quenching such as, for example, a water quench. Generally, quenching is carried out at a rate in the range of about 200C per second to 400C per second. The quenched single phase alloy is a supersaturated single-phase solid solution.

The quenched solid alloy can be directionally aligned by a number of conventional methods which allow passage of the quenched single phase solid through a thermal gradient in a single direction at a fixed velocity of transformation to the transformation temperature. Alternatively, the thermal gradient can be moved relative to the quenched solid. Generally, the apparatus is comprised of a heated vertical mold provided with a cooling system at its lower end, means for maintaining the desired thermal gradient and means for pulling the quenched solid through the thermal gradient at the desired fixed velocity of transformation. The rate that the aligned composite is cooled, once it is formed, is not critical. The geometry of the aligned phase in the aligned composite depends upon the specific composition of the alloy and the velocity at which it is transformed. The aligned phase may be in the form of lamellae, rods or fibers. The lower the velocity of transformation, the lower is the temperature of transformation and the finer is the resulting aligned phase in the composite structure.

In carrying out the present process, the quenched single phase solid alloy is heated unidirectionally through a thermal gradient which achieves cellular precipitation. This is determinable empirically and depends largely on the particular alloy composition. In the present process, the thermal gradient usually ranges from 50C per cm. to about l,000C per cm. For practical purposes the lowest thermal gradient which achieves transformation of the quenched solid is preferred.

The velocity of transformation is determinable empirically and depends largely on the particular alloy composition. Ordinarily, a certain minimum to maximum velocity range will achieve transformation of the quenched alloy into an aligned solid composite. Within the velocity of transformation range, the temperature of transformation increases with increasing velocity. Specifically, fixing of the velocity of transformation also fixes the temperature of transformation in a given system. ln the present invention the temperature of transformation is at least 60C below the solvus temperature of the alloy, and it is a minimum of 20C below the precipitation or transformation temperature of the normal down temperature technique at equivalent velocities in the range from the minimum up to 75 percent of the maximum velocity of transformation. Specifically, at the minimum velocity of transformation in the present process, the temperature of transformation is at least 80C below that of the precipitation temperature of the conventional down temperature transformation technique at the same-velocity. However, with increasing velocity the temperature of transformation increases, and at a velocity above 75 percent of the maximum velocity of transformation in the present process, the difference between the present transformation temperature and that of the precipitation temperature of the conventional down temperature transformation technique is less than 20C and at the maximum velocity of transformation in the present process, the difference in such temperatures is zero.

In a preferred embodiment of the present invention the quenched single phase solid alloy is cold-worked to increase the driving force for the transformation, thereby significantly increasing the obtainable transformation rates. For example, the alloy can be worked at room temperature by methods such as rolling and swaging. The degree of cold-working to achieve a significant increase in the transformation rate is determinable empirically. Generally, a significant increase in the velocity of transformation is attained after the quenched single phase solid alloy is cold-worked in an amount ranging from 1 to 90 percent with increasing amounts of cold work usually resulting in increased velocities of transformation. However, amounts of cold work in excess of 90 percent are not suitable since such amounts inhibit the attainment of a suitably aligned composite product. Specifically, the rate of transformation can be increased by at least percent by such cold-working and such rate of transformation is at least 10 percent higher than that possible by the normal down temperature-transformation technique.

The invention is further illustrated by the following examples.

EXAMPLE 1 A number of samples of Au-40 wt.% Ni alloys are prepared. This alloy undergoes a cellular precipitation reaction wherein the high temperature solid phase precipitates a second solid phase at a solvus temperature of 812C.

Each alloy sample, preferably made from elements of 99.99 percent purity, is formed into a rod 0.175 in. in diameter. Each rod is heated in an atmosphere in which it is substantially inert, such as argon, to a temperature above the solvus temperature to convert it to the high temperature single solid phase, for example to 900C for at least about 1 hour. Each sample can then be rapidly quenched by immersing it in 25C water, which is at a rate of about 400C per second, to retain this high temperature solid phase at room temperature.

To carry out the directional transformation the quenched sample is placed in a graphite crucible, for example, 5 in. long with 0.250 in. outer diameter and 0.035 in. walls and can be directionally aligned in a suitable apparatus where each is driven at constant velocity through a temperature gradient of preferably 300C/cm. Specifically, each quenched sample is pulled under a substantially inert atmosphere such as flowing argon in, for example, a vertical platinumwound furnace and the aligned or transformed portion of the sample can be cooled by driving the crucible upwards through a 541 inch hole in a water-cooled copper toroid.

An insulated chromel-alumel thermocouple can be imbedded in the center of a sample and moved with the sample during pulling and alignment to determine the temperature of tranformation, i.e. the temperature at which the quenched sample directionally transforms to produce an aligned two phase solid composite. The samples driven in the range of 5 X 10 cm/sec. to 6 X 10' cm/sec. will form aligned composites at temperatures of transformation ranging from about 570C to about 600C.

The resulting aligned samples can be polished for metallographic examination and etched for electron microscopy used a solution of CrO in HCl.

The aligned composites are of a substantially uniform microstructure composed of one phase in the form of lamellae substantially parallel to each other grown substantially parallel to the thermal gradient and passing through a matrix of the second phase. At lower temperatures of transformation the lamellae are significantly finer than at higher temperatures of transformation.

EXAMPLE 2 A sample is prepared as set forth in Example 1 except that it is cold-rolled in an amount of about 5 percent before alignment. It is aligned as set forth in Example 1 except that it has a velocity of transformation range significantly higherthan that of Example 1.

In copending US. Pat. application, Ser. No. 416,255 (RD-5715) entitled Directional Eutectoid Composites By Solid-State Up-Transformation filed of even date herewith in the name of James D. Livingston and assigned to the assignee hereof there is disclosed a process of heating a eutectoid type alloy to a temperature above its eutectoid temperature to form the high temperature single solid phase, quenching the single solid phase to retain it at about room temperature, heating the quenched single phase solid unidirectionally through a thermal gradient at a velocity of transformation which on reaching transformation temperature directionally transforms it into an aligned composite of at least two phases with one of the phases aligned in the form of substantially uniform lamellae, fibers or rods substantially parallel to each other and to the thermal gradient.

What is claimed is:

l. A process for producing a solid composite of at least two different metal phases with one phase in the form of substantially uniform parallel lamellae, rods or fibers passing through a matrix of the second or other phases which comprises providing an alloy which forms at an elevated temperature a single solid phase that precipitates a second solid phase when cooled through a solvus temperature, said alloy having a precipitation temperature that varies with velocity of motion down a temperature gradient, heating said alloy to a temperature above the said solvus temperature to form said single solid phase, quenching the resulting single phase alloy to retain said phase at about room temperature, unidirectionally raising the temperature of the resulting quenched single solid phase alloy through a thermal gradient of at least 50C per cm. at a velocity of transformation which on reaching transformation temperature directionally transforms said quenched solid to produce an aligned solid composite of at least two phases wherein one of said phases is aligned in the form of substantially uniform lamellae, fibers or rods substantially parallel to each other and to the thermal gradient,

cent. 

1. A PROCESS FOR PRODUCING A SOLID COMPOSITE OF AT LEAST TWO DIFFERENT METAL PHASES WITH ONE PHASE IN THE FORM OF SUBSTANTIALLY UNIFORM PARALLEL, RODS OR FIBERS PASSING THROUGH A MATRIX OF THE SECOND OR OTHER PHASES WHICH COMPRISES PROVIDING AN ALLOY WHICH FORMS AT AN ELEVATED TEMPERATURE A SINGLE SOLID PHASE THAT PRECIPITATES A SECOND SOLID PHASE WHEN COOLED THROUGH A SOLVUS TEMPERATURE, SAID ALLOY HAVING A PRECIPITATION TEMPERATURE THAT VARIES WITH VELOCITY OF MOTION DOWN A TEMPERA!URE GRADIENT, HEATING SAID ALLOY TO A TEMPERATURE ABOVE THE SAID SOLVUS TEMPERATURE TO FORM SAID SINGLE SOILD PHASE, QUENCHING THE RESULTING SINGLE PHASE ALLOY TO RETAIN SAID PHASE, QUENCHING THE RESULTING SINGLE PHASE ALLOY TO RAISING THE TEMPERATURE OF THE RESULTING QUENCHED SINGLE SOLID PHASE ALLOY THROUGH A THERMAL GRADIENT OF AT LEAST 50*C PER CM. AT A VELOCITY OF TRANSFORMATION WHICH ON REACHING TRANSFORMATION TEMPERATURE DIRECTIONALLY TRANSFORMS SAID QUENCHED SOILD TO PRODUCE IN ALIGNED SOILD COMPOSITE OF AT LEAST TWO PHASES WHEREIN ONE OF SAID PHASES IS ALIGNED IN THE FORM OF SUBSTANTIALLY UNIFORM LAMELLAE, FIBERS OR RODS SUBSTANTIALLY PARALLEL TO EACH OTHER AND TO THE THERMAL GRADIENT, SAID TRANSFORMATION TEMPERATURE BEING A MINIMUM OF 20*C BELOW DAID PRECIPITATION TEMPERATURE WHEN SAID VELOCITIES ARE EQUIVALENT IN THE RANGE FROM A MINIMUM VELOCITY UP TO 75 PERCENT OF THE MAXIMUM VELOCITY OF TRANSFORMATION TO PRODUCE SAID ALIGNED SOLID COMPOSITE.
 2. A process for producing an aligned composite according to claim 1 wherein prior to said unidirectional raising of the temperature said quenched single phase solid is cold-worked in an amount sufficient to raise the maximum velocity of transformation by at least 10 percent. 